The present invention relates to a heat treating apparatus for heat treating objects to be processed, such as semiconductor wafers.
Heat treating apparatuses are generally used in subjecting objects to be processed to a required heat treatment in a soaked state to form thin films on the surfaces of the objects to be treated.
The conventional heat treating apparatus of this type will be explained with reference to FIG. 7. The heat treating apparatus 2 comprises an inner tube 4 of quartz, and an outer tube 6 disposed conically with the inner tube 4 and having a larger outer diameter than the inner tube 4. A plurality of sheets of semiconductor wafers W as objects to be treated are loaded into and unloaded out of the processing vessel, horizontally mounted at a vertical interval on a wafer boat 10 of quartz which is moved up and down by an elevator 12.
A stainless steel manifold 18 is connected to the underside of the processing vessel 8 having a processing gas feed pipe 14 and an exhaust pipe 16. A stainless steel cap 20 is connected to the opening in the bottom of the manifold 8 and is capable of air-tightly closing the bottom opening. The wafer boat 10 is mounted on the cap 20 through a heat insulating cylinder 21 of quartz.
A heating unit 22 is wound longitudinally on the exterior of the circumferential wall of the processing vessel 8 for heating the processing vessel 8. The exterior of the heating unit 22 is covered with a stainless steel outer shell 26 through a heat insulating material 24.
Temperature control in the heat treatment of semiconductor wafers W is a very important factor. Accordingly for precise temperature control, a heating zone is transversely divided into a plurality of zones, e.g., five zones of a first to a fifth heating zones 32A-32E. Temperature sensors, e.g., outer thermocouples 30A-30E are provided in the heating unit 22 corresponding to the respective heating zones 30A-30E. Based on detected values of the temperature sensors, electric power of the heating unit 22 can be controlled for the respective heating zones 30A-30E.
Because of some distance between the wafers W and the heating unit 22, it is difficult for the outer thermocouples 32A-32E of the heater to detect correct temperatures. To compensate for their incorrect detection to some extent, an L-shaped thermocouple tube 34 of quartz is provided between the inner and the outer tube 5 and removably supported by the manifold 18. A plurality of inner thermocouples 36A-36E are provided in the thermocouple tube 34 corresponding to the heating zones 30A-30E.
In heat-treating wafers W in such heat treating apparatus, when the wafer boat 10 of the room temperature with a plurality of wafers W mounted on is loaded into the processing vessel 8 heated up to about 400.degree. C., the temperature in the heating furnace lowers. But power of the heating unit 22 is increased to raise the furnace internal temperature to a processing temperature, e.g., 850.degree. C. A period of time necessary to increase and stabilize temperatures is relatively short because of relatively small heat capacities from the top (TOP) zone of the wafer boat 10 to the vicinity of the bottom zone (BTM) thereof, specifically from the uppermost heating zone 30A to the heating zone 30D immediately upper of the lowermost heating zone, and is relatively easy to control.
But the BTM zone, specifically the lowermost heating zone 30E needs a relatively long period of time to increase and stabilize temperatures because the heat insulating cylinder 21 as a heat insulating material has a large heat capacity, and heat release from the heating furnace opening is large.
To shorten a period of time necessary for the temperature stabilization, a temperature control makes a temperature increase rate of the heating zones as high as possible, and stabilizes a temperature without occurrences of overshoots, etc. To this end, not only temperatures of the internal thermocouples 32A-32E of the heating unit 22 but also temperatures of the internal thermocouples 36A-36E in the heating furnace are monitored to control a temperature of the heating unit 22, based on detected values of the external thermocouples 32A-32E or of the internal thermocouples 36A-36E so that the internal thermocouples 36A-36E have a set temperature.
But in the above-described heat treating apparatus, the internal thermocouples 36A-36E are provided between the inner tube 4 and the outer tube 6 for the temperature compensation. But the internal thermocouples 36A-36E are not sufficiently near each other, and the temperature response of the internal thermocouples is not sufficient. Especially the bottom zone having a large heat capacity because of the heat insulating cylinder 21 has inferior temperature response, and it is a problem that it takes a long time before all the wafers W have a stabilized temperature.
In a heat treatment, e.g., depositing a film processing gas introduced through the processing gas feed pipe 14 ascends in the inner tube 4, reaction products are deposited on the surfaces of the wafers W, and used processing gas descends between the inner tube 4 and the outer tube 6 to be discharged through the exhaust pipe 16. But because of the thermocouple tube 34 accommodating the internal thermocouples 36A-36E disposed downstream of the processing gas flow, excess substances after the reaction, e.g., ammonium chloride as a byproduct of dichlorosilane and ammonium in deposition of silicon nitride, lower quality silicon films in deposition of silicon films using silane, and low-quality silicon oxide films in deposition of silicon oxide using TEOS, stay around the thermocouple tube 34 and the connections of the thermocouple tube to the manifold. When the internal thermocouples are dismounted for maintenance, troubles take place; the thermocouple tube is damaged. Washing off the depositions on the thermocouple tube is hard work. The depositions on the thermocouple tube cause thermal troubles, which makes it more difficult to measure correct temperatures of the wafers.
A plurality of thermocouples, e.g., 5 thermocouples have to be used. Wires connected to the internal thermocouples have to be accordingly long, and cost increases are unavoidable.